Using thermal mass desorption and LEED we have studied interactions of H, C12, and F2 with a silicon (100) surface, and exchange reactions of the gases with adsorbates on the silicon (100) surface. Thermal desorption spectra were measured for surfaces dosed with H, C12, and F2 singly and then for surfaces dosed first with a halogen and then atomic hydrogen. Finally, the reverse sequence was studied, with atomic hydrogen dosing and then the halogen exposure. Results indicate that the molecular halogens C12 and F2 are not effective at removing H from a Si (100) surface. However, for the reverse reaction, atomic hydrogen appears quite effective at removing the halogens.